Small was born at Gibraltar in Clarendon, Jamaica, the daughter of a sugar plantation overseer. Like many Jamaican singers of the era, her career began by winning the Vere Johns Opportunity Hour talent contest, which she won at the age of twelve. Wishing to pursue a career as a singer she moved to live with relatives in Love Lane in Kingston. In her teens, she recorded a duet with Owen Gray ("Sugar Plum") in 1962 and later recorded with Roy Panton for Coxsone Dodd's Studio One record label as 'Roy and Millie'. They had a local hit with "We'll Meet".
These hits brought her to the attention of Chris Blackwell who became her manager and legal guardian, who in late 1963 took her to Forest Hill, London, where she was given intensive training in dancing and diction. There she made her fourth recording, an Ernest Ranglin rearrangement of "My Boy Lollipop", a song originally released by Barbie Gaye in late 1956. Released in March 1964, Small's version was a massive hit, reaching number two both in the UK Singles Chart and in the US Billboard Hot 100, and number three in Canada. It also topped the chart in Australia. Initially it sold over 600,000 copies in the United Kingdom. Including singles sales, album usage and compilation inclusions, the song has since sold more than seven million copies worldwide. Her later recordings, "Sweet William" and "Bloodshot Eyes", also charted in the UK, at numbers 30 and 48 respectively,  and "Sweet William" also peaked at number 40 in the US, her only other American chart single. "My Boy Lollipop" re-charted in the UK in 1987 at no. 46.
"My Boy Lollipop" was doubly significant in British pop history. It was the first major hit for Island Records (although it was actually released on the Fontana label because Chris Blackwell, Island's owner, did not want to overextend its then-meagre resources; in the US, the record appeared on the Smash Records subsidiary of Mercury Records), and Small was the first artist to have a hit that was recorded in the bluebeat style (she was billed as "The Blue Beat Girl" on the single's label in the US). This was a music genre that had recently emerged from Jamaica, and was a direct ancestor of reggae.
On 6 March 1965, Small appeared on the Australian television programme Bandstand. This was as part of a concert at the Sidney Myer Music Bowl in Kings Domain, Melbourne, part of the Moomba Festival. She performed "My Boy Lollipop", "What Am I Living For" and "See You Later, Alligator". Small continued to tour and perform up to the early 1970s.
On 6 August 2011, the 49th anniversary of Jamaica's independence, the Governor-General created Small a Commander in the Order of Distinction, for her contribution to the Jamaican music industry. The award was accepted on her behalf by former Prime Minister Edward Seaga.
In July 2012 she stated that she had been recording again and planned to perform in Jamaica for the first time in over 40 years.
After nearly forty years away from the limelight and refusing all interview requests, Small granted U.S. journalist Tom Graves the first in-depth interview ever in the August, 2016 edition of Goldmine. Previous interviews in the 1960s were typically short and based on press releases. In the interview she discusses fully her early career and the full impact of "My Boy Lollipop." She also insisted, against the denials of Rod Stewart, that it was Stewart who played harmonica on "My Boy Lollipop." She claims to remember the sessions well and recalls Stewart being asked to play.
She had a brief relationship with Peter Asher of the 1960s duo Peter & Gordon. However, in her August 2016 interview with U.S. journalist Tom Graves she denied there had been anything other than a platonic relationship.
|1963||"Don't You Know" / "Until You're Mine"||Fontana|
|1964||"My Boy Lollipop" / "Something's Gotta Be Done"|
|"Sweet William" / "Oh, Henry"|
|"I Love The Way You Love" / "Bring It On Home To Me"|
|1965||"I've Fallen In Love With A Snowman" / "What Am I Living For"|
|"See You Later, Alligator" / "Chilly Kisses"|
|"My Street" / "It's Too Late"|
|"Bloodshot Eyes" / "Tongue Tied"|
|1966||"My Street" / "Mixed Up, Fickle, Lonely, Self-Centred, Spoiled Kind Of Boy"||Brit./Atco|
|"Killer Joe" / "Carry Go Bring Come"||Fontana|
|1967||"You Better Forget" / "I Am In Love"||Island|
|"Chicken Feed" / "Wings Of A Dove"||Fontana|
|1968||"When I Dance With You" / "Hey Mr. Love"|
|1969||"Readin' Writin' Arithmetic" / "I Want You Never To Stop"||Decca|